The present invention relates to devices for measuring electrical current flowing through a conductor, and more particularly to such apparatus which include a magneto-optical element that makes use of the Faraday effect.
Very large electrical currents typically are measured by an apparatus which does not have to be electrically connected to the circuit through which the current flows. One type of such a current measuring apparatus transmits a polarized beam of light through a transparent medium that exhibits the Faraday effect. Because of that effect, the plane of polarization of the light beam rotates by an amount proportional to the magnetic field passing through the substance parallel to the optical axis of the light beam. Magneto-optical materials exhibiting the Faraday effect are commercially available.
U.S. Pat. No. 5,051,577 discloses an example of a current sensor that takes advantage of the Faraday effect in which an optical fiber is wound in a coil around the electrical conductor carrying the current to be measured. A beam of polarized light is transmitted from one end of the fiber to the other end where the polarization of the light is detected. The detection end of the fiber typically is connected to an optical splitter which divides the beam into two orthogonally polarized component beams. Specifically, the polarization axis of one of the output beams was at an angle of +45.degree. to the polarization axis of the input light into the optical fiber, and the other output beam was polarized at an angle of -45.degree. with respect to the polarization axis of the input beam. By detecting the relative intensity of the two output beams, the magnitude of the rotation due to the Faraday effect can be determined. That amount of rotation corresponds to the magnitude of current flowing through the conductor.
Such magneto-optical current sensors have several advantages in that their speed of response allows relatively fast current pulses to be measured. Furthermore, the sensors are constructed from di-electric materials, which is very important when operating at high voltage or in the presence of substantial electro-magnetic interference. However, previous magneto-optical current sensors were relatively expensive requiring optical fibers which can be wrapped around the conductor. Furthermore, because the relatively long optical fiber carried a light beam having a small cross sectional area, laser light sources were used to generate a beam for transmission through the optical fiber. The narrow output beam that emerged from the fiber necessitated an expensive splitter to divide the output beam into two beams of orthogonal polarization. It is therefore desirable to eliminate the use of as many of the expensive sensor components as possible.